1. Field of the Invention
This invention relates to antenna systems which electronically scan antenna arrays and, in particular, to such systems which have independently controllable beams and nulls in their sensitivity patterns.
2. Description of the Prior Art
Antenna assemblies comprising arrays of antenna elements coupled to beam forming apparatus are well known, particularly in the radar and acoustics fields. Such assemblies have steerable sensitivity patterns having high sensitivity portions called beams and low sensitivity portions called nulls. In use it is often desirable to individually steer the beams and nulls. Such a system is disclosed in "DICANNE, A Realizable Adaptive Process," by V. C. Anderson, pp. 398-405. The Journal of the Acoustical Society of America, Vol. 45, No. 2, 1969 and also in "II Sonar Systems Technology," by A. A. Winder, pp. 308-312, I.E.E.E. Transactions on Sonics and Ultrasonics, Vol. SU-22, No. 5, September 1975.
The assembly disclosed in the above-identified articles comprises an array of antenna elements, a steerable beamformer which when connected to the elements establishes a sensitivity pattern containing the steerable beam, and a processor having n input ports and n output ports connected between the elements and the steerable beamformer. The processor comprises a nonsteerable beamformer and delay units. The delay units are connected between the processor input ports and the nonsteerable beamformer for steering a beam in the direction of the null. The output of the nonsteerable beamformer is subtracted from the inputs applied to the processor with the results thereof applied to the processor output ports, respectively. This, in turn, results in a null in the system sensitivity pattern. For some applications, however, the width of the null is less than desirable.